1. Technical Field of the Invention
The present invention relates to the field of integrated circuit, and more particularly to mask-programmed read-only memory (mask-ROM).
2. Prior Arts
Three-dimensional mask-programmed read-only memory (3D-MPROM) has the potential to replace DVD and Blu-Ray Discs. It is ideal for mass publication. U.S. Pat. No. 5,835,396 discloses a 3D-MPROM. It is a monolithic semiconductor memory. As illustrated in FIG. 1, a typical 3D-MPROM comprises a semiconductor substrate 0 and a 3-D stack 10 stacked above. The 3-D stack 10 comprises M (M≧2) vertically stacked memory levels (e.g. 10A, 10B). Each memory level (e.g. 10A) comprises a plurality of upper address lines (e.g. 2a), lower address lines (e.g. 1a) and memory cells (e.g. 5aa). Each memory cell stores n (n≧1) bits. Memory levels (e.g. 10A, 10B) are coupled to the substrate 0 through contact vias (e.g. 1av, 1av′). The substrate 0 comprises transistors. Transistors and their interconnects form a substrate level 0K, which comprises a peripheral circuit for the 3D-MPROM. Hereinafter, xM×n 3D-MPROM denotes a 3D-MPROM comprising M memory levels with n bits-per-cell (bpc).
3D-MPROM is a diode-based cross-point memory. Each memory cell (e.g. 5aa) typically comprises a diode 3d. The diode can be broadly interpreted as any device whose electrical resistance at the read voltage is lower than that when the applied voltage has a magnitude smaller than or polarity opposite to that of the read voltage. The memory level further comprises a data-coding layer, whose pattern is data-pattern and represents data stored in the data-coding layer. In FIG. 1, the data-coding layer 6A in the memory level 10A comprises a blocking dielectric 3b. It blocks the current flow between the upper and lower address lines. Absence or existence of a data-opening 6ca in the blocking dielectric 3b indicates the state of a memory cell. Besides the blocking dielectric 3b, the data-coding layer 6A could also comprise a resistive layer (referring to U.S. patent application Ser. No. 12/785,621) or an extra-dopant layer (referring to U.S. Pat. No. 7,821,080).
In many applications, writing custom data to a 3D-MPROM is desired. Hereinafter, custom data refers to the data which could be unique for different users. For example, to improve the data security, it is desired to store different encryption keys in different 3D-MPROM dice, even though they store the same contents (referring to U.S. patent application Ser. No. 13/027,274). In prior arts, this is implemented with an electrically-programmable read-only memory (EPROM) 0e or laser-programmable fuse. However, EPROM requires a high programming voltage and extra process steps, which increases the wafer cost. On the other hand, a laser-programmable fuse occupies a large silicon area, typically ˜100 μm2. Moreover, the laser-programmable fuses are only formed on the wafer surface. Its data can be easily accessed. As a result, the data security will be compromised.